The present invention relates to an improvement to the drying step utilized in a method for producing ceramic articles, earthenware, bricks or the like, wherein the drying is achieved by means of a stream of hot air in a continuous tunnel drier or in a static drier.
The term "hot air" used in the rest of this description and in the claims refers to the medium for drying the ceramic articles. The medium can be preheated air, hot gases, hot combustion vapours or the like.
A method for producing ceramic articles, such as earthenware, bricks or the like, substantially comprises the following steps in succession: moulding the clay material, drying the moulded articles and firing the articles after drying.
The clay is moulded in conventional devices, e.g. presses or extruders, by a "wet" method using vapour if required, or by a dry method wherein clay is moulded using the moisture in the material itself, combined with adequate moulding pressure. The ceramic articles, irrespective of the manner in which they have been moulded, must be dried for a suitable period in order to eliminate most of the water mixed therewith or hygroscopically absorbed therein. This operation is necessary to ensure that the green articles become sufficiently compact and strong to withstand the subsequent manipulation and loading into the furnaces without being deformed and without cracking, shrinking or breaking, which they would tend to do if the water was removed too quickly from the green ceramic articles, i.e. if they were placed in the furnaces immediately after being moulded.
The ceramic articles can be dried naturally or artificially. Natural drying, brought about simply by the atmospheric air around the ceramic articles, is not desirable due to the high cost of storing and taking out the articles and the fact that the production of finished articles is dependent on the weather. Artificial drying is brought about in static chamber driers or, more commonly, in tunnel driers. The medium used for drying the ceramic articles in the desired manner is a stream of hot air, generally circulated in counter-current relationship with the articles.
The prior-art drying of ceramic articles has technical and economic disadvantages which have not heretofore been overcome. In order to produce the desired heat-exchange conditions and remove the boundary layer of vapour surrounding the ceramic articles, a considerable amount of air has to be conveyed inside the drier, and the hot drying air has to move at high speed. This results in considerable energy consumption required in driving the fans outside or inside the drier, in order to ensure the required flow rate of the hot drying air.
Furthermore, the temperature difference between the hot drying air and the ceramic articles to be dried is still rather small. The reasons are as follows:
(1) The temperature of the ceramic articles entering the drier is relatively low.
(2) The heat exchange between hot air and the articles to be dried occurs in counter-current relationship, and
(3) At the end of the drying process the articles can withstand only a small amount of surface heating, since they are in the phase during which water is diffusing in vapour form into a porous material and the volume of vapour is about 10.sup.3 times the volume of liquid water. Consequently, the ceramic articles will burst if they are surface-heated above a predetermined maximum value.
However, drying is mainly due to heat exchange by forced convection, the maximum exchange being dependent on the speed and amount of hot air being circulated.
Another technical disadvantage is that, at the temperature of the ceramic entering the drier (25.degree.-40.degree. C.), the water in the form of moisture in the green ceramic articles is very viscous and there is a high bonding force between the water and the clay constituting the ceramic articles. As a result, the surface of the articles is dried and shrinks, whereas the interior remains moist. The difference between the interior and the shrinking exterior of each ceramic article produces high tensions in the clay and may result in permanent deformation, microscropic cracking or breakage of the ceramic articles. The danger of micro-cracking is increased by the phenomenon known as thermo-osmosis, consisting mainly of migration of liquid water from the hot surface to an internal, colder region of each ceramic article being dried.
The invention is based on the problem of providing a method of producing ceramic articles, earthenware, bricks and the like, wherein the step of drying the articles is carried out so as to simultaneously to obviate all of the disadvantages of the prior art, so that the ceramic articles can be dried in much shorter times than those at presently required in corresponding known methods.